1. Field of the Invention
This invention relates to a molecular beam epitaxy (MBE) method for the epitaxial growth of Group III nitride semiconductor materials, in particular for the epitaxial growth of an (In,Ga)N quantum well structure or multiple quantum well structure.
2. Description of The Related Art
The InxGa1-xN (0≦×≦1) material family will be referred to herein as “(In,Ga)N” for convenience. The term “InGaN” will be used to denote a member of the (In,Ga)N family having an indium mole fraction that is non-zero, but that is less than one.
The epitaxial growth of Group III nitride semiconductor materials on a substrate can be effected by molecular beam epitaxy (MBE) or by chemical vapour deposition (CVD) which is sometimes known as Vapour Phase Epitaxy (VPE).
CVD (or VPE) takes place in an apparatus which is commonly at atmospheric pressure but sometimes at a slightly reduced pressure of typically about 10 kPa. Ammonia and the species providing one or more Group III elements to be used in epitaxial growth are supplied substantially parallel to the surface of a substrate upon which epitaxial growth is to take place, thus forming a boundary layer adjacent to and flowing across the substrate surface. It is in this gaseous boundary layer that decomposition to form nitrogen and the other elements to be epitaxially deposited takes place so that the epitaxial growth is driven by gas phase equilibria.
In contrast to CVD, MBE is carried out in a high vacuum environment. In the case of MBE as applied to the (In,Ga)N system, an ultra-high vacuum (UHV) environment, typically around 1×10−3 Pa, is used. A nitrogen precursor is supplied to the MBE chamber by means of a supply conduit and species providing gallium and/or indium, and possibly also a suitable dopant species, are supplied from appropriate sources within heated effusion cells fitted with controllable shutters to control the amounts of the species supplied into the MBE chamber during the epitaxial growth period. The shutter-control outlets from the effusion cells and the nitrogen supply conduit face the surface of the substrate upon which epitaxial growth is to take place. The nitrogen precursor and the species supplied from the effusion cells travel across the MBE chamber and reach the substrate where epitaxial growth takes place in a manner which is driven by the deposition kinetics.
At present, the majority of growth of high quality nitride semiconductor layers is carried out using the metal-organic chemical vapour deposition (MOCVD) process. The MOCVD process allows growth to occur at a V/III ratio well in excess of 1000:1. The V/III ratio is the molar ratio of the group V element to the Group III element during the growth process. A high V/III ratio is preferable, since this allows a higher substrate temperature to be used which in turn leads to a higher quality semiconductor layer.
At present, growing high quality nitride semiconductor layers by MBE Is more difficult than growing such layers by MOCVD. The principal difficulty is in supplying sufficient nitrogen during the growth process, and it is difficult to obtain a V/III ratio of 10:1 or greater during MBE growth of a nitride semiconductor layer. The two commonly used sources of nitrogen in the MBE growth of nitride layers are plasma excited molecular nitrogen or ammonia.
One particular application of InGaN layers is in the manufacture of light-emitting diodes and laser diodes that emit light in the blue-green region of the spectrum. Light-emitting diodes and laser diodes that emit in this spectral range can be fabricated using layer structures of group III-nitride semiconductors, as disclosed by S. Nakamura and G. Fasol in “The Blue Laser Diode”, Springer-Verlag (1997). (In,Ga)N quantum well structures are an essential component in the active region of these light-emitting diodes and laser diodes.
The (In,Ga)N quantum well structures in commercially available light-emitting diodes or laser diodes that emit in the blue-green region of the spectrum are grown using the MOCVD growth technique. The commercial growth of such laser diodes or light-emitting diodes by MOCVD has been reported by Nakamura and Fasol (above), and by S. Keller et al in “Applied Physics Letters” Vol. 68, p3147 (1996).
There have been some reports of the successful MBE growth of (In,Ga)N quantum well structures. For example, the growth of InGaN quantum well structures by MBE at a growth temperature of 670° C. has been reported by H. Riechert at al in “Proceedings of International Conference and Nitride Semiconductors”, Tokushina, Japan (1997), and by W. D Herzog et al in “Applied Physics Letters” Vol 70, p1333 (1997). In both these reports, a plasma source was used to generate the activated nitrogen necessary for the growth process.
N. Grandjean et al have reported, In “Applied Physics Letters” Vol. 74 p3616 (1999), the MBE growth of (In,Ga)N multiple quantum well structures using ammonia as the source of nitrogen. However, this growth process was carried out at a low growth temperature, in the range 570 to 620° C. A low growth temperature was obtained because the flux of ammonia to the growth chamber was low, leading to a low V/III ratio.
As a consequence of the low growth temperature, the quality of the InGaN layers grown by these prior art MBE methods is much lower than the quality of InGaN layers grown by MOCVD.
U.S. Pat. No. 5,602,418 discloses a method of growing a multi-crystalline layer on a substrate, so as to allow a single crystal layer that is not lattice matched to the substrate to be grown over the multi-crystalline layer. It does not address the subsequent growth of further layers over the single crystal layer. The method is described for a number of material systems, including the growth of an InGaN layer over an R-face sapphire substrate. U.S. Pat. No. 5,602,418 refers to a growth temperature of 700° C. for an InGaN single crystal layer, but gives no directions as to how such a growth temperature could be achieved,